It is known to polymerize ethylene with very highly active catalysts prepared from organometallic compounds of elements of the I, II and III groups of the Mendelyeev periodic system, and preferably of aluminum, and from the product obtained by supporting a titanium compound on a magnesium halide in active form. Examples of such catalytic systems are reported in GB Pat. No. 1,292,853; U.S. Pat. Nos. 4,089,808 and 4,298,718.
The supported catalytic systems of the above indicated type give, in some cases, ethylene polymers with relatively narrow molecular weight distribution (MWD).
For some applications, however, and in particular for preparing formed articles by injection molding, the molecular weight distribution should be particularly narrow.
A very narrow MWD is also an important requisite of the copolymers of ethylene with alpha-olefins having middle and low density, obtained by low pressure processes (Linear Low Density Polyethylene, LLDPE), in particular as far as the mechanical and optical properties of films are concerned.
Furthermore, in the technology for LLDPE, it is important also that the catalysts, besides allowing to obtain a rather narrow MWD, be capable of providing polymers with a density generally lower than 0.930 containing relatively small percentages of polymerized alpha-olefins. Furthermore, the copolymers should not contain a significant amount of polymer extractable by means of aliphatic solvents.